This invention relates generally to paving material, and more particularly to a method and apparatus for determining liquid absorption of aggregate. Specifically this invention is for determining the saturated, surface-dry state of aggregate and for determining the amount of water and hence binder absorbed by an aggregate in order to determine the proper amount of binder to be added to a given amount of aggregate to produce paving material of acceptable mechanical qualities.
To design an asphalt paving mix, the proper amount of asphalt binder must be added to a given amount of aggregate material to maintain the right matrix of aggregate and binder in order to produce a paving material which will yield a strong and durable road. If there is too much binder in the mix, the road will be soft and rutting will occur. If there is not enough binder in the mix, the road will be brittle and will crumble or break apart.
Aggregates used in the preparation of asphalt for road construction are tested to determine the amount of asphalt binder that will be absorbed internally into the aggregate when a batch is prepared. When binder is absorbed internally into the porous aggregate, that absorbed binder does not contribute to the effective volume of the asphalt mix. In order to account for this, additional binder must be added, which essentially disappears in the mix. The measurement of the binder absorbed by the aggregate which does not contribute to the volume of the asphalt mix is the percent absorption, by weight, of water absorbed into the aggregate to the weight of the aggregate itself (xe2x80x9cPAxe2x80x9d).
The procedure for testing aggregate for PA is as follows. A sample of the dry aggregate is prepared to a condition where the internal voids are saturated with water, and the surface of the aggregate is dry. This condition is known as the saturated surface dry (xe2x80x9cSSDxe2x80x9d) state. The SSD sample is then weighed. The sample is then dried completely in an oven, and weighed again (dry). The difference between the SSD and dry weights, divided by the dry weight, and multiplied by 100, yields the PA.
Another useful measurement is bulk specific gravity (xe2x80x9cBSGxe2x80x9d). BSG is defined as the mass of the material dry divided by the volume of the material at SSD. To determine the volume of the material at SSD the material in the SSD state is immersed in water and the volume of water displaced is equal to the volume of the material at SSD.
One method for preparing a sample of aggregate to the SSD condition is what is know as the xe2x80x9ctowel dryxe2x80x9d method. In this method totally saturated aggregate, i.e. aggregate wetter than SSD, is patted with a paper towel just to the point where the aggregate surface is dry. This technique is best used for larger aggregate such as that for concrete.
The current preferable method for determining whether aggregate for asphalt or concrete is at SSD is what is known as the xe2x80x9cslumpxe2x80x9d test. In this test, a sample of aggregate is prepared with excess water so that it is wetter than the SSD state. The aggregate is placed into a metal cone, the metal cone is placed atop a non-absorbent surface of a table or bench and the aggregate is tamped down into the cone, through an opening in the tip of the cone, with a metal tamper. With aggregate pieces having water on the surface, i.e. with the aggregate sample being wetter than the SSD state, the cone of aggregate will remain standing when the metal cone is removed. The water between the particles of aggregate holds the aggregate together, due to surface tension. The SSD point is reached when there is a xe2x80x9cslight slumpxe2x80x9d of the aggregate when the metal cone is removed. Once the aggregate sample has been initially prepared to wetter than the SSD state the aggregate is progressively agitated and subjected to warm air flowing over it, repacked into the metal cone and the metal cone removed, until this slight slump occurs. A 500 gram sample is then taken from the SSD aggregate and weighed. The 500 gram sample is then completely dried in an oven and is weighed again. The PA is then computed from the two weights.
There are a number of problems with the slump test. First, the test is subjective. The definition of a xe2x80x9cslight slumpxe2x80x9d will vary from technician testing the aggregate to the next. In addition, while the slump test works fairly well with natural sand, for which the test was originally developed, the test does not work as well for jagged material such as crushed granite and limestone. The crushed materials have a higher angularity (jaggedness) and a higher content of fine material, which packs better in the cone, holding the packed material together better. This requires the material to dry more before exhibiting a xe2x80x9cslight slumpxe2x80x9d, making for an artificially dryer SSD point. On the other hand, a method which could actually measure the presence or absence of water on the surface of the aggregate would give a much more accurate measurement of whether the aggregate was in the SSD state or not and hence produce a much more accurate PA measurement.
Second, when the sample is at a temperature above room ambient, it will continue to lose water weight by evaporation as long as the sample remains on the table or bench. This produces an artificially low PA. Also, the time between reaching SSD and weighing the sample will not be consistent from batch to batch and technician to technician. If the sample could maintain its SSD condition/moisture content from the time that that condition is reached until the sample is weighed then the measurement would be more accurate and repeatable from batch to batch and technician to technician.
Third, as the sample is agitated and dried, the sample will begin to generate dust, which leaves the sample, and thus alters the aggregate constitution. Dust can also adversely effect mechanical parts such as bearings, motors, couplings etc. of the equipment used in the SSD/PA testing, thus contributing to premature failure of same. The dust is also a nuisance to the technicians operating the equipment. It would be desirable to somehow contain the dust generated by the sample during the SSD/PA determination.
The present invention solves the noted problems of the slump test in connection with determining BSG and PA, while providing for the liquid absorbing characteristics of aggregate to be determined. The invention is both method and apparatus for determining liquid absorption of aggregate, for determining the SSD state of an aggregate, and for determining the PA of an aggregate.
The method of determining liquid absorption of an aggregate comprises providing a sample of the aggregate, adding liquid to the sample, subjecting the sample to an infrared light source signal and monitoring an infrared light reflected signal reflected from the sample.
Another method of determining liquid absorption of an aggregate comprises providing a sample of the aggregate, adding liquid to the sample, subjecting the sample to a light source signal, monitoring a light reflected signal reflected from the sample and controlling either addition of liquid to the sample or removal of liquid from the sample as a function of the light reflected signal. Preferably the controlling step is controlling addition of liquid to the sample.
The liquid is preferably water. The light source signal is preferably an infrared source signal and the light reflected signal is preferably an infrared reflected signal.
The method further comprises agitating the sample. One manner of agitation comprises moving the sample in an orbital motion. Another manner of agitation comprises moving the sample in a wobbling motion. A third manner of agitation comprises stirring the sample. Preferably the agitating step comprises a combination of the three, namely moving the sample in an orbital motion, moving the sample in a wobbling motion and stirring the sample.
Preferably liquid is added to the sample only until the reflected light signal reaches a predetermined value indicative of the sample being at the SSD state. The predetermined value of the reflected light signal is determined by averaging the reflected light signal reflected from the dry sample with the reflected light signal reflected from the sample when wetter than the SSD state. The reflected light signal reflected from the dry sample, which can vary from aggregate to aggregate, is measured with the apparatus of the present invention and the value of the reflected light signal reflected from the sample wetter than SSD is a constant for all aggregate.
Weighing the sample in the dry state and in the SSD state enables the technician to additionally determine the PA of the aggregate.
The method may alternatively comprise heating the sample to remove liquid from the sample, with the controlling step controlling removal of liquid from the sample as a function of the reflected light signal by controlling the heating of the sample.
The apparatus of the present invention comprises a support for supporting a sample of the aggregate, a liquid source for adding liquid to the sample, an infrared light source which subjects the sample to an infrared light source signal and an infrared light sensor which senses a reflected infrared light signal reflected from the sample.
The apparatus of the present invention also comprises a support for supporting a sample of the aggregate, a liquid source for adding liquid to the sample, a light source which subjects the sample to a light source signal, a light sensor which senses a reflected light signal reflected from the sample and a processor/controller which controls either addition of liquid from the liquid source to the sample or removal of liquid from the sample as a function of the reflected light signal. Preferably the processor/controller controls addition of liquid from the liquid source to the sample.
The liquid source is preferably a water source, the light source is preferably an infrared source and the light sensor is preferably an infrared detector.
The apparatus further preferably includes an agitator for agitating the sample. The agitator may be a turntable which moves the sample in an orbital motion, a turntable which moves the sample in a wobbling motion or a stirrer which stirs the sample. Preferably the agitator is a combination of all three, namely a turntable which moves the sample in an orbital motion and in a wobbling motion and a stirrer which stirs the sample.
The support is preferably a bowl which contains the sample. The bowl preferably includes an island in the center thereof to direct the sample radially outwardly. The bowl preferably includes a lid thereon. The lid preferably includes a dome offset from the center of the lid. The light source and light sensor are preferably positioned such that the light source signal and light reflected signal pass through the dome normal to a surface of the dome. The light source is preferably an infrared source and the light sensor is preferably an infrared detector. The apparatus preferably includes a cabinet containing the support, the liquid source, the light source and the light sensor. The cabinet preferably includes a door providing access to an interior thereof. A bracket is preferably mounted to an underside of the door, and the light source and light sensor are preferably mounted to this bracket.
The processor/controller preferably processes the reflected light signal as a function of time and controls addition of liquid to the sample such that liquid is added to the sample only until the reflected light signal reaches a predetermined value indicative of the sample being at the SSD state. The reflected light signal reflected from the dry sample, which can vary from aggregate to aggregate, is measured with the apparatus of the present invention and the value of the reflected light signal reflected from the sample wetter than SSD is a constant voltage for all aggregate.
The apparatus may alternatively include a weight indicating device for weighing the sample in the dry state and in the SSD state to thereby additionally determine the PA of the aggregate.
The apparatus may further include a heater for removing liquid from the sample, in which case the processor/controller controls removal of liquid from the sample as a function of the reflected light signal by controlling the heater.
The present invention thus avoids the subjectivity of the slump test, replacing it with a much more scientific empirical test which actually measures the presence or absence of water on the surface of the aggregate. The present invention also avoids the problem of the slump test wherein continual water evaporation, after reaching SSD, produces an artificially low PA, since the moistened aggregate is maintained in a bowl sealed with a lid thereby preventing moisture escape. Further, the bowl with lid sealed thereon eliminates the generation of dust as the initially dry sample is begun to be agitated during initial addition of water to the sample.